DE 198 22 411 A1 discloses an actuator unit of this general type.
Because a vehicle operator is decoupled from the physical generation of brake force in electro-hydraulic brake systems (EHB) and the braking demand is realized electrically ‘by wire’, a travel simulator is used reproducing the tactile pedal feedback which simulates the reaction force of a conventional brake system. In addition, the actuator unit includes means allowing a hydraulic emergency braking operation when the electronics is defective by means of direct application of the wheel brakes (so-called hydraulic fallback mode). Outside the hydraulic fallback mode a request for actuation is detected in the by-wire mode as a result of sensing e.g. an actuating travel of the brake pedal, and separating valves are closed to shut off the direct hydraulic throughgrip in the direction of the wheel brakes. While the braking pressure buildup is initiated in an electrohydraulic fashion, a reaction force is imparted to the vehicle operator due to a volume displacement into the travel simulator corresponding to the actuating movement.
The hydraulic fallback mode is of utmost importance. Air that is drawn into the hydraulic circuit (due to leakages or the like) can impair or prevent the functioning of the hydraulic fallback mode because the pressure fluid when mixed with air) reacts compressibly. The minimum requirement of an emergency braking operation as mandated by law—without servo boosting—is not reached. Any air that enters the system will migrate to the highest points of the brake system. Preferred concentration points are the pressure chambers, what is due to a principally inclined mounting position of the actuator unit in the motor vehicle.
The above-mentioned publication does not disclose measures that would allow eliminating or significantly reduce a malfunction that occurs in by-wire brake systems, which is caused by compressibility (or other contamination).
Patent application DE 10147180.7, which is not prior-published, discloses an electronically controllable brake actuation system with means for the electronically controlled separation of contaminants contained and/or dissolved in the brake fluid.